For context, I believe I have a good use case for installing a mini-split system upstairs in my 3 story colonial. It currently has only 1 AC unit and hybrid heat pump/oil furnace, but the basement is unfinished and no supplies running to it. Once I finish the basement, the current system will be insufficient, or at least inefficient as a 1 zone arrangement for all floors. Even now, the 1st and 2nd levels are not balanced mostly due to inefficiencies and sheer size of the home. 
Enter: Install mini split for upstair bedrooms, and use exiting ductwork in basement to supply it when finishing next year. 
The ductwork is leaky and poorly insulated, and the return duct emits strong musty odors. A lot of humidity in this home that I’m sealing up. Rather than repair, clean, and insulate current ducting and install a traditional system using the existing branches, I want to tear them out of the attic and remove the large return/supply trunks to the attic. A lot of inefficiency and moisture penetration lives here and I also gain more room in a walk in closet as well as attic storage. 
My question comes in when configuring the air handler cassettes (I prefer over wall mounted). I don’t see a need to have true ‘multi-zones’, but I know I need multiple handlers to properly temperature control the upstairs. I’ve read that these cassettes can have ducting branched off for 1/2 of their air supply but haven’t been able to find manuals or diagrams that demonstrate this. Can someone provide their experience or resources that I could read up on? It seems pretty easy.. make sure the air handler BTUs are sized for 2 rooms rather than 1.. and run the duct vent to the other room. This would allow me to only need 3 units for the 1 condenser vs 4 or 5 if I’m unable to branch. 4 bedrooms, master bath, and hall bath with a large landing. LG units currently have a sale until end of Sept. and it’d be sweet to install this before the winter so I know how much more insulation/air sealing I need to do in the attic.